sick cat, please help

I have a 5 yr old domestic shorthair, her name is Luna, she has been sick for about 4 or 5 months. at first, it was just sneezing, then it progressed to bloody sneezes. Took her to vet, We have had several rounds of anti biotics, prednisone. Cat is still sick and getting worse, she has lost weight, won't eat hardly anything, I haven't seen her drink in a few days, She has had xrays, and they flushed her nasal cavity, still nothing the doc wants her to get a ct scan and a probe biopsy at a cost of over 500 dollars. My wife and I are on a very limited fixed income. Poor luna looks as though she is suffering, should we have her put down. Would appreciate advice.